Thread size and pitch, by family: the reference table for the bench

A thread gauge tells you the pitch. Callipers tell you the diameter. Together they will get you to a designation — and a designation is only two thirds of the answer, because the thing that decides whether a joint seals is the seat, and the seat is not something a thread gauge can see.

Every fitting has three independent properties: the thread (size and pitch), the seat (how it seals), and the gender. A designation like 9/16"-18 describes the first only. Two fittings sharing it can be entirely different parts.

Both appear in our catalogue at that size, on dozens of variants each. They will thread together. The cone will bottom against the flat face, the O-ring will have nothing to seal against, and the joint will weep under pressure — usually not immediately, which is what makes it a field problem rather than a bench one.

Designations below are the ones carried across our own catalogue, family by family. Pitch is threads per inch on the imperial families and millimetres on metric.

At 1/2" and 3/4" the pitches match exactly, and the two will engage. They still are not the same thread — NPT has a 60° flank angle and BSP has 55°, so the flanks never seat properly along their length. Forcing one into the other damages the port, and on a cast-iron valve body that is the expensive kind of damage.

The two highlighted rows are the collisions. Everywhere else the families diverge in diameter even where the pitch coincides, so the parts will not start — which is the safe failure. 9/16"-18 and 1.3/16"-12 are the two sizes where the wrong part goes in without complaint.

There is no measured male thread outside diameter column here, and it is the column most people want — the one that turns “I measured 15.8 mm” into a designation. We do not hold that measurement as data, so we are not publishing it. A table of thread diameters recalled rather than measured is precisely the artefact that causes the errors this article is about.

What works instead, today: measure the diameter and the pitch, then send both with a photograph of the end face. The seat is visible in a photograph and it is the property that resolves the JIC and ORFS collision that a diameter alone cannot. Our thread identifier walks the same four questions interactively if you would rather answer them one at a time.

Figures verified 24 August 2026 · Designations and pitches taken from the port labels on our own adapter catalogue, restricted to sizes carried across a substantial number of variants in each family. Thread outside diameters are not published because we do not hold them as verified data.